Combine the cauliflower, onion, and water in a medium saucepan. Cover the pan and then bring it to a boil. Reduce the heat, and then continue to simmer for about 5 minutes, or until the vegetables are crisp and tender. Drain well, and set aside.
Add the milk, cornstarch, bouillon granules, and salt to the saucepan. Beat well with a wire whisk to blend and then bring to a boil. Reduce the heat and simmer, stirring constantly until the mix has thickened and become bubbly.
Add the cheese and stir with a wire whisk until melted. Remove from heat. Add the cauliflower-onion mixture, chicken, and pimento to the cheese sauce.
Spoon the mixture into a 1 1/2-quart casserole dish coated with cooking spray and then bake for 20 minutes, or until heated thoroughly.